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Gas Dynamics of Turbine Cascades
P M V SubbaraoProfessor
Mechanical Engineering Department
Its Group Performance, What Matters.……
Turbine Blade on A Disk
Introductory Remarks
• The science of gas dynamics is becoming more and more applied to turbines.
• It is of interest to inquire how far the knowledge of aerofoil mechanics is applicable to steam and gas turbines.
• Almost all steam turbine blades have much greater curvature than basic aerofoils.
• This is a necessity for utilizing the entire pressure difference of steam/gas in a practicable number of stages.
The Influence of Neighbors
The group of Aerofois
• An important point to be examined is whether the crowding together of steam/gas turbine blades has effects similar to those noticed in aeroplane practice.
• A biplane is stated to have less lifting effect than a monoplane of the same wing surface.
• The biplane arrangement is analogous to the circumferential pitching of steam/gas turbine blades.
• Additional wing surfaces in series (one behind the other), analogous to avoiding of pressure compounding in a turbine.
• The reasons for grouping may have some bearing on turbine blading efficiency.
Influence of Neighboring aerofoils
The Group Behavior
Steps in Analysis of Blade Cascades
Nomenclature of Turbine Cascades
More Geometrical Specifications of Arrangement of A Cascade
Vr1
Vr2
More Geometrical Specifications of Arrangement of A Cascade
Ub
Geometrical Details of A Cascade
The Cascade
Solidity: the first Step in Cascade Design
• One of the important aspects of cascade design is the selection of the blade solidity.
• Defined as the ratio of chord or axial chord to blade spacing.
• A minimum allowable value is usually desired from the standpoint of reducing weight, cooling flow, and cost.
• An increase in the blade spacing eventually results in decreased blade efficiency due to separated flow.
• An optimum solidity cascade should be a fully attached flow with maximum blade spacing.
• The gas dynamic factors affecting solidity are
• The requirements of velocity diagram
• The blade loading